All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea|crustaceans]], and worms. The head projects into a crown of prehensile arms ranging from 8 in octopus to about 90 in the living nautilus. Cephalopod arms, or tentacles, and used for grasping prey, are a specialized development of the basic molluscan foot. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].

All cephalopods are carnivorous, feeding primarily on fish, other [[Mollusca|mollusks]], [[Crustacea|crustaceans]], and worms. The head projects into a crown of prehensile arms ranging from 8 in octopus to about 90 in the living nautilus. Cephalopod arms, or tentacles, and used for grasping prey, are a specialized development of the basic molluscan foot. Once the prey is snared it is bitten with strong beak-like jaws and pulled into the mouth by the [[radula]].

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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician [[Endocerida|endocerid]] [[Nautiloidea|nautiloid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Pachydiscus seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons. Such giant cephalopods play or played a similar ecological role of top predator to that of [[Devonian]] [[Arthrodira|arthrodire]] [[Placodermi|placoderms]], [[Mesozoic]] [[pliosaur]]s and [[Cenozoic]] [[Odontoceti|toothed whales]].

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Cephalopods are quite large by molluscan standards (most species being between 6 and 70 cm including tentacles), with the giants of the group - such as the modern day ''[[Architeuthis]]'', the giant squid, with a body length (including tentacles) of up to 20 meters, the Ordovician [[Endocerida|endocerid]] [[Nautiloidea|nautiloid]] ''[[Cameroceras]]'', with a straight shell up to 10 metres in length, and the Cretaceous [[Ammonoidea|ammonoid]] ''[[Pachydiscus|Pachydiscus seppenradensis]]'', with a coiled shell 3 metres in diameter - the largest invertebrates ever to live, with weights of one to two tons. Such giant cephalopods play or played a similar ecological role of top predator to that of [[Devonian]] [[Arthrodira|arthrodire]] [[Placodermi|placoderms]], [[Mesozoic]] [[pliosaur]]s and [[Cenozoic]] [[Odontoceti|toothed whales]].

Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals). There is a high level of cephalization (development and concentration of sensory and neural centers in the head). The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes. They are visual creatures, changing colour to express mood. The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates. The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.

Cephalopods have a highly developed nervous system, unequalled among the invertebrates, and correlated with locomotor dexterity and carnivorous lifestyle (predators generally always have larger brains than prey animals). There is a high level of cephalization (development and concentration of sensory and neural centers in the head). The nerve ganglia are concentrated and more or less fused to form a brain that encircles the esophagus. A bundle of giant nerve fibres tied to the [[mantle]] give them very rapid reflexes. They are visual creatures, changing colour to express mood. The eyes of the [[Coleoidea]] are very elaborate, with a retinal structure remarkably like that found in vertebrates. The eye of the giant squid is the largest of any animal - 40 cm across. Nautiloids have smaller and more primitive eyes.